Bacteria joined cold and hunger in the defeat of Napoleon's troops in Russia

Napoleon Bonaparte assembled a massive army of nearly 600,000 men in the early summer of 1812 to invade Russia. The Grande Armée reached Moscow at the end of the summer, but found it empty. Isolated and short of supplies due to the Russians' scorched-earth practices, the French chose to retreat to their winter quarters near Poland before the cold trapped them in the city of the tsars. The retreat was the French emperor's greatest military disaster. The extremely low temperatures and lack of food made life easier for pathogens: it is estimated that around 300,000 soldiers died along the way. Now, a study led by researchers at the Pasteur Institute has identified two bacteria in the remains of Napoleonic soldiers. Under normal conditions, they don't kill, but for those unfortunate souls, it was the final straw.

Among the first to enter Moscow was Dr. JRL de Kirckhoff, a physician assigned to the headquarters of the French Third Army Corps. Years later, he would write a book detailing the diseases that plagued the imperial soldiers during their retreat. Specifically, he documented the prevalence of typhus, diarrhea, dysentery, pneumonia, and jaundice. “At that time, it had not yet been discovered that microorganisms could cause infectious diseases, so the description of a disease was based solely on symptoms,” recalls Nicolas Rascovan , head of the microbial paleogenomics unit at the Pasteur Institute and senior author of this research, published in Current Biology .

A group of experts led by Rascovan recovered the remains of 13 soldiers of the Grande Armée buried in Vilnius (Lithuania) along with three thousand others. They were looking for evidence of typhus, which, since Kirchhoff, has been considered the disease that most severely affected Napoleon's retreating troops. To find it, they looked at the teeth of the fallen, which are best preserved in the fossil record: "If the pathogen that infected one of them was circulating in the blood at the time of death, the bacterial DNA would be preserved in the blood that reaches the dental pulp as if it were a blood sample from the individual," Rascovan explains.

The French army suffered almost no casualties in its few clashes with the Russians during the invasion. The extreme conditions and high mortality rate following their retreat from Moscow were captured by Adolph Northen in his painting "Napoleon's Retreat from Russia." Barbieri et al., Current Biology

They found no trace of typhus or any of the other pathologies listed by Dr. Kirckhoff. But the teeth of four of them tested positive for Salmonella enterica Paratyphi C, a member of the salmonella group that causes paratyphoid fever. In two others, they found DNA from Borrelia recurrentis , a bacterium responsible for relapsing fever. Although these two diseases are different, they can cause similar symptoms, such as high fever, fatigue, and digestive problems. As with typhus, relapsing fever is transmitted by body lice, a different species than the one that appears on the heads of many schoolchildren every September. The etiology of paratyphoid fever is different; it can be caused by unsafe water, contaminated food, or contact with feces containing the bacteria.

Under normal conditions, none of these pathogens are fatal. "But if you're on the brink, if you're immunosuppressed, starving, or freezing, any little bug can push you and you'll fall," Rascovan explains. Although salmonellosis due to foodborne illness is very common, relapsing fever disappeared from Europe more than a century ago. Following the work of Louis Pasteur and Robert Koch on the causal connection between many microorganisms and disease, a few hygiene and public health measures were enough to banish a good portion of them.

“There are only seven sequenced genomes of current strains of B. recurrentis . It's so difficult to find that only one study managed to isolate several, all identical,” says Rascovan. “This disease is very difficult to find and is found primarily on the African continent, specifically in the Horn of Africa.” Since the 1980s, this part of the world has been the region hardest hit by famine and war. “Its sanitary conditions are perhaps most similar to those that existed in Europe at the time,” concludes the Pasteur Institute scientist.

The fact that they didn't find DNA from the causative agent of typhus, a bacterium of the genus Rickettsia , doesn't mean that Dr. Kirckhoff was wrong in his diagnosis. In fact, in 2006, after the excavation of the Vilnius mass grave, a dental examination of 35 soldiers (from the same burial site, but different individuals) identified DNA from Rickettsia prowazekii in three of them. In seven others, they found traces of Bartonella quintana, the cause of trench fever. Both bacteria use human body lice as a vehicle of transmission. The authors of this study also located several examples of the parasite among the soldiers' uniforms.

The soldiers' teeth are the key: they have recovered pathogen DNA from dental pulp. In the image, the jawbone of a French soldier next to a button from his uniform. He belonged to the 17th Infantry Regiment. Michel Signoli, Aix-Marseille University

Adding together the teeth analyzed in 2006 and those analyzed now, it turns out that a third of the samples had some pathogen in their bodies. Although the authors are cautious and don't extrapolate, if this percentage were applied not only to the nearly 3,000 of those buried in Vilnius, but also to the other 300,000 who left Moscow but didn't leave Russia alive, infectious diseases were among those that defeated Napoleon Bonaparte in the east, along with General Winter and famine.

“During Napoleon's retreat from Moscow in the winter of 1812, sanitary and living conditions completely collapsed,” recalls Remi Barbieri, the study's first author and a postdoctoral fellow at the Pasteur Institute during the research. “Temperatures dropped below -30°C, food and shelter were almost nonexistent, and exhausted soldiers marched hundreds of kilometers through snow and mud in tattered, louse-infested uniforms,” adds Barbieri, who delved into the history of the retreat. The combination of poor hygiene, hunger, and extreme cold created the perfect breeding ground for epidemics. “Under such desperate conditions, louse-borne and waterborne diseases, such as typhus, relapsing fever, paratyphoid fever, and trench fever, spread rapidly through the ranks,” he explains. “These multiple infections acted in concert, devastating an army already weakened by exhaustion and hunger, and turning the retreat from Moscow into one of the deadliest episodes in military history,” concludes Barbieri, now a researcher at the Institute of Genomics at the University of Tartu (Estonia).

For Francesco Maria Galassi, associate professor of anthropology and paleopathologist at the University of Łódź (Poland), Barbieri and Rascován's work represents a major breakthrough: "Paleogenetic analyses allow us to better understand the role of infectious diseases in major military campaigns, such as the Napoleonic War, and in many other wars throughout history." In fact, Galassi, who was not involved in this work, points out that "even today, infections associated with poor hygiene and the collapse of healthcare systems remain a crucial problem in current conflicts, from the Middle East to Ukraine."